A human ear normally transmits sounds such as speech sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window membrane of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and three quarters turns. It includes three chambers along its length: an upper chamber known as the scala vestibuli, a middle chamber known as the scala media, and a lower chamber known as the scala tympani. The cochlea 104 forms an upright spiraling cone with a center called the modiolus where the axons of the auditory nerve 113 reside. These axons project in one direction to the cochlear nucleus in the brainstem and they project in the other direction to the spiral ganglion cells and neural processes peripheral to the cells (hereinafter called peripheral processes) in the cochlea. In response to received sounds transmitted by the middle ear 103, sensory hair cells in the cochlea 104 function as transducers to convert mechanical motion and energy into electrical discharges in the auditory nerve 113. These discharges are conveyed to the cochlear nucleus and patterns of induced neural activity in the nucleus are then conveyed to other structures in the brain for further auditory processing and perception.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. In some cases, hearing impairment can be addressed by an auditory prosthesis system such as a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processing stage 111 into a digital data format, such as a sequence of data frames, for transmission into a receiver processor in an implant housing 108. Besides extracting the audio information, the receiver processor in the implant housing 108 may perform additional signal processing, and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 112 which penetrates into the cochlea 104 through a surgical opening called a cochleostomy. Typically, this electrode array 112 includes multiple electrode contacts 110 on its surface that deliver the stimulation signals to adjacent neural tissue of the cochlea 104 which the brain of the patient interprets as sound. The individual electrode contacts 110 may be activated using various stimulation strategies that include, for example, sequential or simultaneous stimulation in one or more contact groups.
The representation of temporal information in an auditory system by use of cochlea implants is imperfect compared to a normal functioning hearing organ. In a healthy ear, temporal information is recorded by the hair cells and their corresponding nerve fibers before the information is translated to the brain. Up to a certain frequency, the hair cells can follow the externally generated acoustic information in phase with the corresponding oscillation of the basilar membrane. However, the nerve fibers have a certain refractory period which allows only a limited temporal coding. In case of a healthy physiological system, there are sufficient nerve fibers present having different refractory states after stimulation. Consequently, acting as an ensemble, these nerve fibers together are typically able to represent temporal information up to 5 kHz (see, for example, Wever and Bray's “volley theory,” 1937).
In case of a cochlear implant, the temporal information is provided via the electrodes by, for example, biphasic electrical pulses which directly elicit action potentials in the nerve fibers. As a consequence, all nerve fibers around an electrical contact of the cochlear implant electrode are elicited synchronously and the volley principal is not applicable any more. Transfer of temporal information may thus be strongly impaired.
The coupling to the neuronal system is therefore imperfect with regard to a cochlear implant. Further, conditions like the actual impedance of the cochlear implant's electrode contacts may be different from patient to patient. As a consequence, standardized pulse sequences may be sub-optimal for patients. To help address this problem, psycho-acoustic tests have been performed in which the patient provides subjective feedback whether he is able to discriminate pitch of presented sounds at various pulse rates. (See Bahmer and Baumann 2012, Cochlear Implants International, accepted). However, such subjection feedback can vary depending on the circumstances, and may be hard to achieve with certain patients, for example, small children.